The present invention relates to methods of preparing solid dosage forms using direct compression techniques. In particular, the present invention relates to methods of directly compressing tablets containing relatively high amounts of acetaminophen based on the total tablet weight.
In order to prepare a solid dosage form containing one or more active ingredients (such as drugs), it is necessary for the materials to be compressed into the dosage form possess certain physical characteristics which lend themselves to solid dosage form processing. Among other things, the material to be compressed must be free-flowing, must be lubricated, and, importantly, must possess sufficient cohesiveness to insure that the solid dosage form remains intact after compression.
In the case of tablets, the tablet is formed by pressure being applied to the material to be tableted on a tablet press. A tablet press includes a lower punch which fits into a die from the bottom and a upper punch having a corresponding shape and dimension which enters the die cavity from the top after the tabletting material fills the die cavity. The tablet is formed by pressure applied on the lower and upper punches. The ability of the material to flow freely into the die is important in order to insure that there is a uniform filling of the die and a continuous movement of the material from the source of the material, e.g. a feeder hopper. The lubricity of the material is crucial in the preparation of the solid dosage forms since the compressed material must be readily ejected from the punch faces.
Since most drugs have none or only some of these properties, methods of tablet formulating have been developed to impart these desirable characteristics to the material(s) which is to be compressed into a solid dosage form. Typically, excipients are added to the formulation which impart good flow and compression characteristics to the material as a whole which is to be compressed. Such properties are typically imparted to these excipients via a pre-processing step such as wet granulation, slugging, spray drying, spheronization, or crystallization. Useful direct compression excipients include processed forms of cellulose, sugars, and dicalcium phosphate dihydrate, among others.
Lubricants are typically added to avoid the material(s) being tabletted from sticking to the punches. Commonly used lubricants include magnesium stearate and calcium stearate. Such lubricants are commonly included in the final tabletted product in amounts usually less than 1% by weight.
In addition, solid dosage forms often contain diluents. Diluents are frequently added in order to increase the bulk weight of the material to be tabletted in order to make the tablet a practical size for compression. This is often necessary where the dose of the drug is relatively small.
Another commonly used class of excipients in solid dosage forms are binders. Binders are agents which impart cohesive qualities to the powdered material(s). Commonly used binders include starch, and sugars such as sucrose, glucose, dextrose, and lactose.
Disintegrants are often included in order to ensure that the ultimately prepared compressed solid dosage form has an acceptable disintegration rate in an environment of use (such as the gastrointestinal tract). Typical disintegrants include starch derivatives and salts of carboxymethylcellulose.
There are three general methods of preparing the materials to be included in the solid dosage form prior to compression: (1) dry granulation; (2) wet granulation; and (3) direct compression.
Dry granulation procedures may be utilized where one of the constituents, either the drug or the diluent, has sufficient cohesive properties to be tabletted. The method includes mixing the ingredients with a lubricant, if required, slugging the ingredients, dry screening, lubricating and finally compressing the ingredients.
The wet granulation procedure includes mixing the powders to be incorporated into the dosage form in, e.g., a twin shell blender or double-cone blender under shear mixing conditions and thereafter adding solutions of a binding agent to the mixed powders to obtain a granulation. Thereafter, the damp mass is screened, e.g., in a 6- or 8-mesh screen and then dried, e.g., via tray drying or fluid-bed drying. The wet granulating technique is rather time consuming due to its process steps and can also be considered to be relatively expensive. In addition, wet granulating has been known to reduce the compressibility of some pharmaceutical ingredients including microcrystalline cellulose.
Direct compression, on the other hand, is regarded as a relatively quick process wherein the powdered materials included in the solid dosage form are compressed directly without modifying their physical nature. Usually, the active ingredient, direct compression vehicle and other ancillary substances, such as a glidant to improve the rate of flow of the tablet granulation and lubricant to prevent adhesion of the tablet material to the surface of the dies and punches of the tablet press, are blended in a twin shell blender or similar low shear apparatus before being compressed into tablets. This type of mixing of the ingredients was believed to be essential in order to prepare pharmaceutically acceptable dosage forms. For example, Remington's Pharmaceutical Sciences, 16th Edition (1980), Arthur Osol, Ed., cautions artisans that the manner in which a lubricant is added to a formulation must be carefully controlled. Consequently, lubricants are usually added to a granulation by gentle mixing. At page 1556, Remington's, warns: "Prolonged blending of a lubricant with a granulation can materially affect the hardness and disintegration time for the resulting tablets." Further, those of ordinary skill in the art have long believed that excessive mixing of a lubricant with the granulate ingredients overcoats the granules and reduces the tablet hardness or tablet strength of the compressed tablets. Thus, for at least these reasons, high shear mixing conditions have not been used to prepare direct compression dosage forms.
Pharmaceutical manufacturers would often prefer to use direct compression techniques over wet or dry granulation techniques because of its processing time and cost advantages. Direct compression, however, is usually limited to those situations where the drug or active ingredient has a requisite crystalline structure and the physical characteristics required for formation of a pharmaceutically acceptable tablet. Often, however, one or more excipients must be combined with the active ingredient before the direct compression method can be used since many active ingredients do not have the necessary properties. Since each excipient added to formulation necessarily increases the tablet size of the final product, artisans were often limited to using direct compression techniques in formulations containing a rather low load of active ingredient per compressed tablet. Solid dosage forms containing the drug to be administered in a relatively high load or dose (e.g., the drug itself comprises a substantial portion of the total compressed tablet weight), could only be directly compressed if the drug itself had sufficient physical characteristics (e.g., cohesiveness) for the ingredients to be directly compressed.
For example, acetaminophen, a widely used analgesic, is considered to be a high load active ingredient. Most commercial compressed tablet formulations include anywhere from 70 to 85% by weight acetaminophen per finished tablet. This high load of active ingredient combined with its rather poor physical characteristics for direct compression have not allowed pharmaceutical manufacturers to use direct compression techniques to prepare the final tablets. Previous attempts to directly compress acetaminophen with microcrystalline cellulose have failed to provide an acceptable product. The final products tend to be soft, prone to capping and otherwise not commercially desirable, i.e., difficult to swallow because of the large size. Consequently, the more time consuming and expensive wet granulation techniques must be used.
Thus, another limitation of direct compression as a method of tablet manufacturing is the potential size of the compressed tablet. If the amount of active ingredient is high, a pharmaceutical formulator may choose to wet granulate the active with other excipients to attain an acceptably sized tablet with the desired amount of acetaminophen. Usually the amount of filler/binder or excipients needed in wet granulation is less than that required for direct compression since the process of wet granulation contributes to some extent toward the desired physical properties of a tablet.
Microcrystalline cellulose, a processed cellulose, has been utilized extensively in the pharmaceutical industry as a direct compression vehicle for solid dosage forms. Microcrystalline cellulose is commercially available under the tradename EMCOCEL.RTM. from Edward Mendell Co., Inc. and as Avicel.RTM. from FMC Corp. Compared to other directly compressible excipients, microcrystalline cellulose is generally considered to exhibit superior compressibility and disintegration properties as long as it is not wet granulated prior to compression.
Thus, despite the advantages of direct compression such as reduced processing times and costs, wet granulation is widely used in the industry to prepare solid dosage forms. Currently, many skilled in the art also prefer wet granulation over direct compression because wet granulating has a greater probability of overcoming any problems associated with the physical characteristics of the various ingredients in the formulation, thereby providing a material which has the requisite flow and cohesive characteristics necessary to obtain an acceptable solid dosage form.
The popularity of the wet granulation process as compared to the direct compression process is based on at least three advantages. First, wet granulation provides the material to be compressed with better wetting properties, particularly in the case of hydrophobic drug substances. The addition of a hydrophilic excipient makes the surface of a hydrophobic drug more hydrophilic, easing disintegration and dissolution. Second, the content uniformity of the solid dosage forms is generally improved with the wet granulation method because all of the granules obtained thereby usually contain approximately the same amount of drug. Thus, segregation of the different ingredients of the material to be compressed (due to different physical characteristics such as density) is avoided. Segregation is a potential problem with the direct compression method. Finally, the particle size and shape of the particles comprising the granulate to be compressed are optimized via the wet granulation process. This is due to the fact that when a dry solid is wet granulated, the binder "glues" particles together, so that they agglomerate in the granules which are more or less spherical.
In spite of the advantages afforded by wet granulation methods, many manufacturers would nonetheless welcome the opportunity to directly compress tablets containing acetaminophen, especially those containing high loads of acetaminophen and/or microcrystalline cellulose.
Thus, there still remains a need in the industry for techniques and pharmaceutical excipients which would allow artisans to prepare direct compressed dosage forms containing relatively high amounts of acetaminophen by weight and thereby avoid the time and expense of wet granulations.